In processing of coal and potassium chloride it is typical to use a one or more solid-liquid separation stages in order to isolate the desired solids. In all of these mining processing applications, the finest fractions of solid-liquid slurries may be subjected to screen bowl centrifugation for recovery of dewatered solids. The screenbowl centrifuge in a coal or potassium chloride processing plant is used to remove the moisture from the coal or potassium chloride. As part of the moisture removal some of the solids, usually about −40 microns in size, are lost.
The screenbowl centrifuge rotates at a very high rpm. The slurry of coal or potassium chloride enters through the feed pipe and discharges inside the machine. The solids accelerate to the high rpm portion of the machine and thus encounter high centrifugal forces. This causes the solids to move to the outside of machine and the solids are then moved toward the discharge by an auger. The liquid moves in the opposite direction and discharges as “effluent” at the “bowl drain”. The effluent is typically sent to waste and thus the useful coal or potassium chloride in the effluent is lost. That is the waste that mining processing is trying to reduce. A small fraction of the liquid leaves the centrifuge at the screen drain, however, that is not the fraction of interest in this work because the amount of screen drain liquid is relatively small compared to the effluent and the screen drain liquid typically is recycled back through the process.
The separation inside the machine is made at about 40 microns; greater than about 98% of the +40 micron material reports to the discharge as product along with about 50–70% of the −40 micron material. The remaining about 30–50% of the −40 micron solids report to the effluent and are discarded.
In these separations, it is always a goal to increase the amount of useful coal and potassium chloride that is recovered.
In the past when flocculants were added to the feed prior to entering the machine, the high centrifugal forces inside the machine resulted in the flocculants being sheared, and thus they were not effective in capturing more solids. Therefore, it is widely accepted in the industry that flocculent addition is not effective in reducing the amount of solids being discharged in the effluent.
It would be desirable to identify materials capable of flocculating coal and potassium chloride within the high shear conditions of a centrifuge separation.